Position measuring devices, e.g., absolute position measuring devices, may have a backup current source in order to prevent the loss, in case of an interruption of the main current source, of data important for the operation of the encoder, such as data indicating the rotational position of a motor or the position of motion.
For example, in the position measuring device illustrated in FIG. 4, there are a primary current source 3, a backup current source 1a (external battery) and a chargeable high capacitance capacitor C1 inside an encoder 2. When primary current source 3 is interrupted, the current required is backed up or buffered by backup current source 1a and high capacitance capacitor C1. That means, at normal operation, a voltage Vcc from primary current source 3 drives a current via nodes n1 and n2 and a diode D24 to the internal circuits. In this context, high capacitance capacitor C1 connected to node n1 is also charged. If an interruption of primary current source 3 occurs, a voltage Vbat drives a current from a battery BAT on the inside of backup current source 1a via nodes n3 and n4 to principal encoder part 2, and via a diode D23 a current supply of internal circuits 23 is undertaken. Since diode D24 is present on the side of primary current source 3, no reverse current of the backup current occurs in this context.
On the inside of backup current source 1a, a normal battery BAT is available, and during an exchange of the battery on account of the service life of this external battery BAT, after a sufficient charging of high capacitance capacitor C1, primary current source 3 is switched off and the external battery current source is exchanged during buffering by the high capacitance capacitor within a period of time during which buffering by high capacitance capacitor C1 is possible.
Such encoders are frequently used for the position report of servomotors, and, because of technical revolution in recent years, downsizing and, on account of the problem of heat development of the motor because of downsizing, high temperature capability is called for in the operating temperature range. However, when the high temperature capability is considered in the operating temperature range, the problem may arise that the buffering capacity decreases in the course of time based on the influence of the heat-conditioned quality decrease of the capacitor, and the buffering time may be reduced. As a high capacitance capacitor for buffering, an electric double layer capacitor may be used, and since this may be extremely large compared to general electronic components, a disadvantageous influence on the downsizing of the encoder or on the layout room on a printed circuit board may be unavoidable.
As regards such backup current sources, for example, in Japanese Published Application No. 2000-014047, an encoder is described that has the task of making available an encoder and its backup method, the capacitor for buffering at a location distant from the encoder being developed in an environment having a different temperature, and the reduction in quality of the capacitor is prevented and the reliability of the buffering activity is increased, and, in addition, limitations with respect to the shape and the dimensions of the encoder itself are removed, and downsizing may be achieved. The encoder described in this publication, in the case of which, after the processing of the detection signal of an encoder detection part for determining rotations or a straight-line motion by electrical circuits connected to the primary current source, an encoder output signal is emitted and at least the encoder detection part and the electrical circuits are disposed in an encoder housing, is arranged such that the buffer capacitor for buffering in response to an interruption in the primary current source is arranged outside the encoder housing, the buffer capacitor being connected to a lead wire extending out of the encoder housing, and being arranged at a location distant from the encoder housing, the lead wire being connected between a diode connected to the primary current source and the electrical circuits, and the buffer capacitor being connected directly to the primary current source.
But, since the encoder described in this publication uses a direct capacitor for buffering the main current source, it may not be practical, since the backup current source may be insufficient and the buffering time may be insufficient. And since, in addition, no distinction is made between the current system fed by the main current source and the current system fed by the backup current source, the current usage may be high even during the buffering, whereby the above problem may become worse, and, in addition, a decrease of the service life of the backup current source itself may occur.
Japanese Published Application No. 2001-309577 describes a backup current supply device that has the task of making available a backup current supply device, the backup battery being able to be safely exchanged without the backup data of an absolute encoder being lost. As far as the construction of the device of this publication is concerned, in this context a backup current supply device is involved in which, in parallel, at least two backup batteries and connecting parts for connecting these batteries are made available, and at least one backup battery is connected to one connecting part, while a new backup battery is connected to the other battery connecting part, so that the backup battery may be safely exchanged and a loss of backup data may be prevented.
However, since even in the device described in this publication no distinction is made between the current system fed by the main current source and the current system fed by the backup current source, the current usage may be high also during the buffering, which is why the problem of the service life of the backup battery may still not be solved.
Japanese Published Application No. 2002-213994 describes a backup current supply device, the task of which is to make available at a favorable price a backup current supply device having excellent reliability and service-proved capability, in which there may be no loss of the data of the absolute encoder of a servo system that are to be buffered. As far as the construction of the device of this patent literature is concerned, a servomotor on which an absolute encoder is arranged, a motor control which controls the servomotor and a backup current supply device is made available. The backup current supply device includes a buffer capacitor connected in parallel to a backup battery. In response to the interruption of the main current source of the absolute encoder, the backup current supply device undertakes the buffering of the data. It is connected to the main current source via a detachable connecting terminal or clamp, so that the exchange of the backup power supply device may be easily possible. Furthermore, in parallel, at least two second connecting terminals are made available for connecting the backup current supply device, and the connecting terminal of the backup current supply device is connected to at least one of the second connecting terminals. Since, during the exchange of the backup current supply device, the new backup current supply device is able to be connected to the second connecting terminal, at which there is no connection, there may not come about any loss in the data that are to be buffered.
However, just as with the publications mentioned above, since no distinction is made between the systems of the main current source and the backup current supply device, the current drain may be high, and similar problems may appear as in the above examples.